Why is the event bubbling triggered twice?

Event bubbling may be triggered twice because of the binding method of the event processing function, event delegation, event object methods, nested relationship of events, etc. Detailed introduction: 1. How to bind event processing functions. When binding event processing functions, you can use the "addEventListener" method to bind events. If the same type of event processing functions are bound to the same element multiple times, Then during the event bubbling stage, these event processing functions will be triggered in sequence, causing the event to be triggered multiple times; 2. Event delegation is a front-end development technique and so on.

The operating system for this tutorial: Windows 10 system, DELL G3 computer.

In front-end development, event bubbling (Event Bubbling) is a stage of the DOM event model. It means that during event propagation, the event bubbles up from the target element that triggered the event until it reaches the top-level element. During the event bubbling phase, event handling functions are executed in order from inside to outside.

The event bubbling mechanism is to facilitate developers to handle multiple elements during event propagation. When an element triggers an event, such as a click event (click), the event will first be triggered on the triggering element, and then bubble up level by level, triggering the event handler on each ancestor element in turn.

However, sometimes we may encounter a situation where event bubbling is triggered twice. This may be due to the following reasons:

1. Binding method of event processing function:

When binding event processing function, we can use the addEventListener method to bind the event. If the same type of event handling function is bound to the same element multiple times, then during the event bubbling stage, these event handling functions will be triggered in sequence, causing the event to be triggered multiple times. Therefore, when binding event handling functions, you need to ensure that you only bind them once, or unbind existing event handling functions at the appropriate time.

2. Event Delegation:

Event delegation is a common front-end development technique. It uses event bubbling mechanism by binding event processing functions to parent elements. to handle events on child elements. In event delegation, if the same type of event handling function is bound to both the parent element and the child element, then during the event bubbling stage, these event handling functions will be triggered in sequence, causing the event to be triggered multiple times. Therefore, when using event delegation, you need to ensure that the event handler is only bound to the parent element to avoid repeated triggering.

3. Methods of event objects:

The event object is a parameter passed in the event processing function. It contains event-related information and methods. In the event handling function, we can control the behavior of the event through some methods of the event object. For example, you can stop the bubbling propagation of an event by calling the stopPropagation method of the event object. If the event object's methods are not used correctly in the event handler, the event may bubble up and be triggered multiple times.

4. Nesting relationship of events:

In front-end development, there may be multiple nested elements, that is, one element contains another element. If the same type of event handling function is bound to both the parent element and the child element during the event bubbling stage, then during the event bubbling stage, these event handling functions will be triggered in sequence, causing the event to be triggered multiple times. Therefore, when processing events of nested elements, you need to pay attention to the binding and triggering order of event processing functions to avoid repeated triggering.

It should be noted that the event bubbling mechanism is based on the specification of the DOM event model, and different browsers may have different implementation methods. Therefore, when writing front-end code, you should try to follow the standard DOM event model and conduct compatibility testing to ensure the consistency and reliability of the code in different browsers.

In summary, in front-end development, event bubbling is a stage of the DOM event model. It means that during the event propagation process, the event starts to bubble upward from the target element that triggered the event, and is triggered in sequence. Event handler function on each ancestor element. Event bubbling may be triggered multiple times due to reasons such as the binding method of the event processing function, event delegation, methods of the event object, or the nested relationship of the event. Therefore, when writing front-end code, you need to carefully consider all aspects of event handling to ensure that event propagation and processing behavior are as expected.

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